Voltage fault detectors are known which sample an AC voltage source a plurality of times during each cycle. U.S. Pat. No. 4,475,047 discloses a system for disconnecting a first voltage source from a load and connecting a second voltage source to the load upon the detection of a fault condition. The system of U.S. Pat. No. 4,475,047 samples the amplitude of the AC signal from the first voltage source a plurality of times during each cycle. A counter stores a word GOODBD whose cumulative value indicates if the amplitude of the first voltage source is acceptable by counting samples of the AC signal from the first voltage source which are within an acceptable range. A number equal to or greater than 5 indicates that the input AC signal is within acceptable limits. The counter is augmented each time the detection of the sampled waveform within an acceptable range occurs. A further counter counts a word BADCOUNT whose value is used to open a line switch when its cumulative value is equal to zero. BADCOUNT is decremented each time the AC input voltage is found not to be within acceptable limits U.S. Pat. Nos. 3,505,598, 4,087,697, 4,156,280, 4,219,858, 4,219,860, 4,356,553 and 4,423,458 disclose systems for sampling the voltage of AC voltage sources.
U.S. Pat. No. 4,446,498 discloses a system for monitoring current flow to a load which accumulates and decrements a count proportional to current flow for purposes of generating a trip signal when the count reaches a predetermined value.